Pipeline pigs are used to inspect, maintain and clean pipelines such as those used in oil-and-gas, chemical, and water distribution industries. The pigs include circumferential sealing elements arranged about the central body or mandrel of the pig. The sealing elements, which are typically made of an elastomeric material, use product flow to drive the pig forward under differential pressure while maintaining sealing engagement with an inner wall surface of the pipe. To help ensure this sealing engagement, the seal elements are sized for the constant internal diameter section of pipeline the pig is to be used in and tight clearances are required. If the next section of pipeline has a different (smaller) diameter cases, a different size pig must be used for the next section of pipeline.
Alternative types of dual-diameter pigs which can traverse different pipeline percentage changes include foam pigs, butterfly pigs, paddle pigs, and spring suspension pigs (see Dave Bacon, Pushing the Boundaries of Dual Diameter Pig Design (Pigging Products and Services Assoc., 2002), herein incorporated by reference). The operating envelope for each type of pig is limited (see e.g. FIG. 1) as are the pipe diameters the pigs can function in. For example, a dual diameter pig can function in two different diameters but not a diameter in between those two.
Foam pigs have dual diameter capability by default, but this capability is very limited. Butterfly pigs make use of slotted guide discs that collapse when in the smaller diameter line and another set of elements that provide sealing and drive functions in that line. Paddle pigs collapse into the smaller diameter line by rotation induced by chamfers on the paddle, with sealing provided by buckle-inducer type discs in the larger diameter line and by small diameter seals in the smaller line. Spring suspension or wheel pigs makes use of spring-loaded arms with wheels to center the pig and buckle-inducer type discs and small diameter seals for sealing in the larger and smaller diameter lines, respectively.
Therefore, a need exists for a multi-diameter pipeline pig that can traverse dual- and multiple-diameter pipelines with the same set of sealing elements providing sliding seal engagement with the inner wall surface of the pipe in the different diameter pipes and performing the primary function of drive or cleaning within those different diameter pipes.